What would a pile of neutrons look like?

Let’s say I had a device that could slow down free neutrons to less than a few mm/s. I hook that up to a neutron emitter until I have a few kilograms of neutrons. Not enough to make neutronium, just enough to make a pile big enough to play around with.

Just how exactly would a pile of neutrons look and behave? Would it look like a transparent gas? A opaque gas? A liquid? a thick layer of dust? If I disturb it, would it take a little time to settle or a long time to settle? Could I keep my neutrons in a glass bottle or would they all leak out the bottom? Is there any material that could contain it? Would it atomically react with any element it settles on and form isotopes would would it be fairly non reactive? Would it be safe to run my hands through it? To breath it in?

The main thing is that they are unstable. They decompose is a fairly short period of time, somewhere around 10 minutes (I forget offhand what the halflife is). In the meantime though, they’d be transparent, since they have no electrons to interact with light. Not sure about other properties, although I suspect there’s no container that’d hold them, for the same reason.

Loose neutrons don’t ‘look’ like anything because they don’t interact with light; it’s the electrons around the nucleus in normal matter that make it possible to see. It would be a transparent gas that would probably pass through most matter - electron interactions again are what make objects solid in day to day life. Because of that I’m not sure that you could move it or breathe it in, I think you’d just pass through it, unless you were using something like a thick lead glove.

It wouldn’t be a bunch of loose neutrons for long, though - neutrons outside of a nucleus are unstable and undergo beta decay with a half life of about ten minutes. So after playing with your neutron ‘pile’ for ten minutes, you’d get a high dose of radiation and half of it would turn into protons (or ionized hydrogen), and it would be pretty much gone in an hour. Definitely not something you want to hang around with if you ever plan on having kids or want to avoid cancer.

Radioactive decay is continuous. You’d get a sizable dose right from the start, not just at ten minutes.

Hmm, tricky. Let’s assume you have some way to keep them cool, which is not going to be easy, since they will be decaying immediately and emitting fairly substantial amounts of high-energy radiation, and as soon as any protons form (from the decay) any collisions between them and neutrons will start forming atomic nuclei, with additional large releases of energy, on the fusion scale. You need some way to immediately get rid of the protons before they can have any collisions with the neutrons. And actually the neutrons themselves will, as soon as they collide, also stick together with fusion-level releases of energy. It’s hard to see how this experiment lasts long enough for any light emission or absorption, which is what you’re after.

If we could somehow turn off the strong force, so they just bounced off each other and did not decay, then I guess we could observe them. (They bounce off each other for the same reason atoms bounce off each other, which has nothing to do with any force, but is rather a consequence of the Pauli exclusion principle, which says two fermions (e.g. the electrons in two different atoms, or two neutrons) can’t be in the same place in the same state, which essentially means with similar energy.)

So what we’ve got is a cold neutron gas in a box. I’m going to guess it would look like a silvery-white translucent gas. What you have in this case is something like what a metal looks like: a collection of particles in a box. There will be a very large number of “particle in a box” translational states, very closely spaced (because this is a macroscopic box). The neutrons will fill them up to some level, the Fermi level, because they are fermions, just like the electrons in a chunk of metal fill up the conduction band to the Fermi level. The gas can absorb and re-emit (i.e. scatter or reflect) light at pretty much any wavelength, because the levels above the Fermi level are still very closely spaced, so almost any size quantum of energy can be absorbed. This is what happens in a metal, and it’s why metals have a silvery-white appearance.

Now, one complication is that neutrons have no charge, so any absorption of photons has to go through the much weaker interaction of the electromagnetic field with the neutron’s magnetic dipole moment (this is similar to the way nuclei absorb radio waves in a nuclear magnetic resonance experiment). Hence I would guess that even with all those available energy levels the intrinsic absorption cross section will be very low – hence, translucent silvery-white gas.

I guess if the temperature is low enough and we continue to magically turn off the strong force, we might get a liquid, held together by weak magnetic dipole interactions. Whether it could crystallize at zero pressure (1 atm is zero as far as atomic energy scales go) I have no idea. Like He, it might be necessary to apply a little pressure even as T->0 to get crystallization. One imagines it would then look like the ground state of the 3D Ising model, all the spins lined up.

Well right there your device is poorly defined… the neutrons are highly unstable and they are irradiating everything and releasing enormous amounts of energy… so you won’t feel a neutron, well you may feel pin prick sized feelings if you were only feeling individual neutrons hitting you, until after a while you notice bleeding and burning and inflammation … depending on the rate of neutrons… But basically if the neutrons are stopped, then the heat will build up locally, making it rather hot… but that means its destroying itself, its contradictory… the air will get hot. meaning that it starts up convection and blows the neutrons away…

Unless of course your device is storing the neutrons at the pressure of the insides of a neutron star… Where the answer becomes “your hand transforms into neutrons”.

So really the only answer is that a single neutron decay probably has the energy of a pin prick, like a grain of sand fired at 50 km/h hitting you… You know at the beach, getting sand blasted … its like an individual grain of that … but its too small to just notice one or two… its not significant enough.

If the results of the neutron radiation are kept in the area, the apparatus must be becoming radioactive… you don’t feel the radiation causing radiation burns … so its not like you can feel if its safe or not.

It has everything to do with the strong force. Yes, yes, two particles can’t be in the same state… but how close counts as “the same state”? What determines just what states are available to a particle? That’s all a function of what forces are acting on the particles. And when two particles “bounce off of each other”, that’s always because of one of those forces, not from some “Pauli exclusion principle force”.

Also, “sufficiently cold” in this context means “less than about a billion K”.

Yeah, I should have said ‘while playing with your neutrons, over the first ten minutes…’ since it’s a gradual release not a sudden burst right at ten minutes. You couldn’t poke them for five minutes then leave and be safe.

Why is a cloud of atomic particles being called “a gas” here?

Serious question. Gas is a definable phase of matter, no? So what are the satisfied criteria here?

Large number of particles bouncing off one another with elastic collisions. So kinetic energy and momentum preserved. Pretty well all you need to create a cloud of something that obeys PV = nRT.

Lots of things can be gasses. The defining characteristic is that the particles spend most of their time basically as free particles, with interactions only when the particles are very close together.

If you found something that could mechanically confine a “neutron gas”, I expect it would approximately obey the ideal gas law, probably even better than molecular gasses do.

a pile of neutrons would look like spots in the eye… the radiation hits the retina… spot spot spot…

They can be bottled. The first cite from '79 is behind paywall, but the tiltle and blurb say yes, they can be contained in a metal bottle. The other cite mentions an experiment on neutron half-life using a bottle (although the thrust is the disagreement with another methodology).


I didn’t know this. So how long does it take for a typical neutron star to decay away?

Thanks on “gas” query.

Free neutrons decay, but neutrons in a bound state don’t—basically, the resulting state (containing an electron and a proton as decay products) would be of higher energy than the original state (with the undecayed neutron), and thus, the decay can’t occur (or rather, it occurs only up to the point where adding a new proton/electron would take more energy than is released in the neutron decay).

Being overly pedantic and snarky about an ancillary detail of a short post answering a physics question on a message board doesn’t make you look as clever as you think it does, especially when you didn’t even attempt to answer the OP’s question yourself. You are technically correct, which according to pedantic bureaucrats is the best kind of correct.

Please accept my non-apology for answering the OP’s question in simple language and only referring to the bound state that can safely exist on earth, and not mentioning neutron stars since they’re not really relevant to what the OP asked.

What if I were to put my junk in that box?

Neutrons happily interact with other matter and will bounce off of nuclei of other atoms, regardless of electrons being there or not, so they won’t pass through most matter. They will certainly penetrate into the material and scatter around until they lose enough energy and the capture cross section rises so that they are captured by the material.
For example wafting a lead glove through a cloud of fast neutrons is not going to do much. The neutrons will bounce off the lead nuclei and not lose much energy in those collisions, so leaving the cloud of neutrons in very much the same state as before you wafted the hand through.
If however you were to pass your ungloved hand through the cloud of fast neutrons, you would actually slow a lot of them down, due to all the hydrogen nuclei in all the water in your hand. The hydrogen nuclei are the same mass, almost, as the neutrons, so for each collision the neutrons will lose a lot more energy than if it hit a lead nuclei. This slowing down from fast to epi thermal and thermal speeds means the neutrons are much more likely to be captured by the chlorine and other elements in your hand. So an unshielded hand will actually interact more with the neutrons than the proposed lead shielded hand.